1. Field of the Invention
The present invention relates to an apparatus and method for sensing hydrostatic pressure, in a distributed fashion, in hydrocarbon pipelines and wells. The invention is particularly concerned with the detection and localization of flow restrictions and blockages in hydrocarbon flowlines for flow assurance purposes.
2. Background of the Invention
As part of the overall process of oil extraction and processing, it becomes necessary to transport the fluids containing liquid hydrocarbons from their reservoirs to remote plants for chemical processing. This transport process is usually conducted and pipelines that can be anywhere from several hundred meters to various kilometers in length. Ensuring the safe, reliable and continuous transport of hydrocarbons through the pipelines is of vital importance to oil companies and hydrocarbon refineries. It is also equally important to properly measure the pressure, flow and composition of the produced fluid. However, the transport of liquid hydrocarbons is faced with serious problems such as the buildup of wax and scale in the pipe walls, internal pipe erosion and corrosion, formation of hydrates and asphaltenes, and several others.
It is well understood that fluid containing heavy hydrocarbons tend to precipitate and form waxy crude oils as they flow through pipelines. These paraffin precipitates deposit on the inner walls of pipes accumulating over time and forming a solid layer that narrows the passage of any liquid flow. In addition, other chemicals present such as sulfates, calcium carbonate, drilling fluids, and other scale precipitates, start depositing solid debris layers that further obstruct the fluid flow. Obviously, if one were to know the mechanisms of wax formation, it would be possible to predict the time at which a dramatic flow reduction would occur. This is in practice possible, but in order to make the analytical models accurate and effective, it is necessary to have accurate measurements of deposited wax thickness. This is not an easy task to perform on operational flowlines and most methods available are based on invasive or destructive techniques to arrive at the sought wax thickness value.
Ensuring pipeline safety and reliability, and the flow assurance of hydrocarbons are the main drivers for the development of new, on-line, monitoring techniques for the detection and localization of wax and hydrate build-ups and blockages in oil pipelines. Pipeline blockages have dramatic operational and economic consequences. For example, consider an oilfield with 8 wells, each producing 10,000 barrels of oil per day (B/d). The importance of operating at peak efficiency of transport within a pipeline is demonstrated by considering that a 5% increase in efficiency—for a pipeline transporting 80,000 B/d of crude—would result in an increase of 4,000 B/d in transported oil. This would translate to an annual revenue increase of $36 million, assuming $25/barrel. Furthermore, as oil production practices move to regions with deeper reservoirs and cold waters, these problems become more serious, and thus it becomes increasingly important to develop monitoring systems that alert operators when the conditions are critical for wax and paraffin formation to occur.
In general, there are two popular approaches to dealing with this problem: chemical injection and pigging. In the case of injection, chemical inhibitors are injected inside the pipeline to prevent the formation of, or dissolve any wax or hydrate build-up. Pigging consists in the mechanical removal of deposited wax and build-ups inside pipelines via a mechanical swab element commonly known as a “pig”. The pig is commonly inserted inside the flowline through an access port, and pushed forward by fluid pressure or some other mobile mechanism. As the pig moves, it scrapes the inner surfaces of the conduit, removing any wax or scale build-up present. For instance, in U.S. Pat. No. 6,615,848, Coats illustrates the use of an electronically controlled pig element that is buoyant and able to travel inside pipelines. The pig has provisions for the measurement and removal of build-up and avoids damage to the interior pipe walls by the use of a selectively expandable body. However, both the above-mentioned approaches are expensive and cumbersome, and they also require periodic maintenance and calculated guessing on the part of the operators to determine an appropriate time to conduct the chemical injection or pigging operations. Chemical injection also carries the risk of contaminating produced water, restricting its release to the sea.
In an effort to reduce the costs associated with ensuring the flow of produced fluids from the wellhead to the primary processing facility based on the above techniques, the oil industry has shown increasing interest in reducing wax build-up and in on-line monitoring instrumentation. One approach is to use electrical heating and insulation of long and deep flowlines, to prevent hydrate or wax formation. This technique may be augmented by the use of a distributed fiber optic temperature sensor to help obtain temperature profiles of the flowline and detect the onset and location of possible blockages, as well as cold temperature zones along the flowline. However, for this approach to become practical, it becomes necessary to have access to the pipeline prior to its deployment in order to install the necessary electrical heating conductors and associated monitoring optical fibers and thermal insulation.
Other on-line monitoring systems rely on the non-intrusive detection of blockages via acoustic or strain measurements taken from the outside of the flowline. U.S. Pat. No. 6,513,385 describes an acoustic sensor based on a piezoelectric transducer that emits an acoustic pulse signal. The pulse traverses the pipeline walls as well as the various deposited layers until it impinges on the opposite side wall, where the pulse is reflected back to the transmitter. Wax build-up layers are detected by measuring the time delays between incoming and returning pulses arriving at the acoustic transmitter. As before, one problem here is the fact that sensor heads need to be installed and secured around the flowlines. This presents difficulties for retrofitting into existing subsea installations. In addition, the technique might not be effective until a certain wax layer thickness is developed and, often times, it becomes necessary to calibrate and couple the system to a particular pipe and build-up combination.
Berthold et al., in U.S. Pat. No. 5,845,033, describe a fiber optic blockage detection system based on an array of fiber Bragg grating strain sensors disposed along a continuous fiber length. The sensor arrayed is mounted or spirally strapped around the exterior of a pipeline so that there is good mechanical transfer of the pipe stresses to the fiber. Any internal pressure change resulting from a flow restriction or blockage will result in a hoop strain that can be detected by the fiber Bragg grating strain sensors. As before, this approach assumes that the fiber installation can be accomplished prior to the pipeline installation itself. In addition, proper mechanical bonding and strain transfer between the pipeline and optical fiber control the efficacy of the technique. Any unwanted stress in the pipe which is not directly the result of an internal hydrostatic pressure change, can give rise to an erroneous reading or false blockage detection.